1. Field of the Invention
The present invention relates to a voltage controlled oscillator.
2. Description of the Related Art
As various kinds of electronic devices have come to be more compact, the downsizing of a voltage controlled oscillator (VCO) mounted in these electronic devices has also been under consideration. For the downsizing of an electronic circuit, it is effective to form components composing the circuit on one chip in an integrated manner, and therefore, forming circuit components composing the VCO on one chip is also a conceivable method also for the VCO.
The structure of the VCO will be described in detail in an embodiment and here a brief description thereof will be given. The VCO includes an inductance element and a capacitor being a capacitance element both of which form a series resonance circuit, and an oscillation frequency of the VCO is decided by a resonance point of the series resonance circuit. The VCO further includes a varicap diode being a variable capacitance element for shifting the resonance point of the resonance circuit. On a subsequent stage of a resonance part including these inductance element, capacitance element, and variable capacitance element, there is provided a transistor serving as an amplifying part and a feedback part including a feedback capacitance element, the amplifying part and the feedback part forming an oscillation loop together with the resonance part. These resonance part, transistor, and feedback capacitance element form the oscillation loop. Incidentally, at the time of the oscillation, the resonance part is inductive in order to generate a negative resistance −Rv to cause the oscillation.
The circuit elements of the resonance part and the feedback part need to be designed according to an oscillation frequency of the VCO, and if a VCO whose circuit elements are all formed as one chip is to be manufactured, it is necessary to make a specialized mask for each type (oscillation frequency) of the VCO, which leads to an increase in manufacturing cost including development cost.
A patent document 1 describes a VCO in which a passive resonator is formed on a substrate separate from an integrated circuit. However, the patent document 1 does not take up the aforesaid problem, and in the disclosed VCO, capacitors C1, C2 being feedback capacitance elements are included in an IC chip, and thus the aforesaid problem is not solved sufficiently.
The following problem also arises when the oscillation frequency becomes high. Specifically, when the frequency becomes high, for example, several GHz or ten several GHz, variable capacitance elements become inductive, and a bonding wire used for the connection of the variable capacitance elements also becomes inductive, and as a result, a variable width (span of adjustable range) of the frequency becomes narrower than a design value or no oscillation occurs. This implies that the manufacture of a VCO in a high frequency band is difficult.    [Patent document 1] Japanese Patent Application Laid-open No. 2000-183647 (FIG. 2 and so on)